Does it seem reasonable to use a small front vise with guide rods mounted under the workbench to use as a wagon vise. This would have the outer face plate mounted to the workbench and not moving and the center of the vise where the drive screw passes through would now move as it is not screwed to the bottom side of the bench top. The back of the guide bars of the shoulder vise would be set into a block and mounted to the bottom of the workbench. A small block mounted to the center now moving section of the vise and rising up through the slot in the workbench top with a dog hole and dog in it to act as the clamping member.
The large front vise with guide rods could also be used to create a leg vise by building a leg around the large shoulder vise mounted vertical in it. The bottom of the leg vise would use one guide roller as in another well known designed leg vise. The use of the large shoulder vise would only need one guide roller at the bottom of the leg vise as the shoulder vise having the long guide rods would help keep it moving smoothly.
Just an idea, any thoughts or other ideas on this. This would give you two vises for less than the cost of one of the popular systems on the market.